This invention relates generally to jet propulsion engines, and more specifically to compressor airfoils used therein.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. The combustion gases are discharged through turbine stages which extract energy therefrom for powering the compressor, and producing output power for use in driving a booster (low pressure compressor) and a fan in an exemplary turbofan aircraft engine application.
A multistage axial compressor includes cooperating rows of stator vanes and rotor blades which decrease in size to pressurize air in stages. The compressor vanes and blades have corresponding airfoils which typically vary in configuration as their size decreases from stage to stage for maximizing performance of the compressor. Compressor performance includes, for example, efficiency of compression, flow capability, and stall margin, which are all affected by the configuration of the vanes and blades.
More specifically, the flow or pressure distribution of the air as it is being compressed through the stator vanes and rotor blades is a complex three dimensional flow field varying circumferentially around the compressor, radially along the span of the vane and blade airfoils, and axially along the circumferentially opposite pressure and suction sides of the airfoils.
The airfoil pressure side is a generally concave surface cooperating with the opposite suction side, which is a generally convex surface, for efficiently pressurizing the air as it flows between blades in the axial downstream direction between the leading and trailing edges thereof. The pressure distribution of the air undergoing compression varies from the radially inner root of the airfoil to the radially outer tip of the airfoil which is spaced closely adjacent to a surrounding compressor casing to provide a suitable radial gap or clearance therewith.
The airfoil, itself, may be supported from the compressor rotor in any suitable manner such as being formed integrally therewith in a unitary blisk configuration, or each rotor airfoil may have an integral platform and dovetail for mounting the compressor blade in a corresponding dovetail slot formed in the perimeter of the compressor rotor.
Axial and mixed flow compressor blades that are designed to compress the air usually have a rotor or number of rotors that rotate inside a stationary casing and act to raise the total pressure and temperature of the flow passing through the machine. The compressor rotor blades carry a lift on the body of the airfoil that manifests itself as a higher static pressure on the pressure surface of the airfoil and a lower static pressure on the suction surface of the airfoil. Generally a small gap exists between the tip of the compressor rotor and the radially adjacent casing flowpath. The pressure difference between pressure side and suction side of the airfoil drives flow through the tip gap of the compressor rotor. This tip flow can roll up into a vortex, which tends to collect on the pressure side surface of the circumferentially adjacent blade, leading to high levels of loss and blockage in the compressor tip region. As this blockage spreads across the compressor rotor tip, the ability of the compressor to produce a pressure rise decreases, and may result in a stall in some cases. This problem is exacerbated by weak flow near the endwalls, caused by cumulative effects of skin friction and secondary flows. Weak endwall flow allows the aforementioned vortices to remain in the rotor tip region, rather than being convected downstream.
In the art, casing treatments, such as circumferential grooves have sometimes been used to control or reduce the tip leakage and improve stall margin, but with an associated efficiency penalty. While these methods serve to reduce tip leakage flow levels, they do not control losses and blockage created by the remaining tip flow. In conventional designs, it is generally accepted that pressure and velocity profiles will be somewhat weak in the endwall region, due to the aforementioned loss mechanisms.
Accordingly, it would be desirable to have a compressor rotor blade having an airfoil with specific features that can reduce the propagation of the flow blockage across the blade passage thereby facilitating improvement of the compressor stall margin. It is desirable to have a compressor rotor blade having an airfoil that can strengthen endwall flow speed and pressure to facilitate reduction of tip blockage and increase throttle margin.